| Characteristics of PsA cases | Monozygotic(n = 12) | Dizygotic(n = 39) | Difference(95% CI) |
| Pairwise characteristics | |||
| Mean age in 2002 in years (range) | 50 (27, 74) | 51 (27, 79) | 1.0 (−6.3, 8.4) |
| Mean discordance time of arthritis (range)* | 8.7 (1, 16) | 13 (1, 42) | 4.4 (−2.2, 11) |
| Mean time of shared environment (range)* | 19.0 (14, 30) | 19 (14, 29) | 0.7 (−0.8, 2.2) |
| Characteristics of twin individuals | |||
| Female proportion | 29% | 52% | 24% (−6%, 46%) |
| Mean age at onset of psoriasis (range) | 29 (11, 50) | 26 (2, 54) | 3 (−5.5, 11) |
| Mean age at onset of arthritis (range) | 37 (20, 59) | 36 (18, 63) | 0 (−8, 8) |
| Mean number of joints involved† | 4.5 | 3.7 | 0.9 (−1.1, 2.9) |
| Proportion with axial disease‡ | 17% | 7% | 9% (−8%, 38%) |
| Proportion with nail involvement | 71% | 50% | 21% (−8%, 44%) |
| Proportion HLA-B13 positive | 0% | 11% | 11% (−12%, 25%) |
| Proportion HLA-B27 positive | 21% | 5% | 16% (−3%, 42%) |
PsA, psoriatic arthritis.
*Calculated on the pairs that were concordant for response. †Number of joints with current or previous arthritis. ‡Axial disease was defined as inflammatory back pain and definite unilateral or bilateral sacroiliitis on x ray (see text).